1. Field of the Invention
The present invention relates to an optical scanning apparatus and an image forming apparatus using such an optical scanning apparatus which are suitably applicable to apparatuses, such as laser beam printers, digital copying machines, and multi-function printers that employ the electrophotographic process, and in which a single or plural modulated light beams from a light source unit is reflectively deflected by a polygon mirror of a deflecting unit, transmitted through a scanning optical system, and scanned on a surface to be scanned (a scanned surface) to record image information, for example.
2. Related Background Art
In a conventional optical scanning apparatus such as a laser beam printer (LBP), a light beam from a light source unit, which is optically modulated according to the image signal, is periodically deflected by a deflecting unit, such as a polygon mirror, is condensed and projected on a surface of a photosensitive recording medium, such as a photosensitive drum, in the form of a spot by a scanning optical system having f-θ characteristics, and is scanned on this surface to record image information.
FIG. 16 illustrates a schematic perspective view illustrating a principal portion of a conventional optical scanning apparatus.
In FIG. 16, a divergent light beam emitted from a light source unit 91 is converted into a nearly parallel light beam by a collimator lens 92, the width of the nearly parallel light beam is restricted by an aperture stop 93, and the light beam is then incident on a cylindrical lens 94 having a predetermined refractive power only in a sub-scanning direction. The nearly parallel light beam incident on the cylindrical lens 94 emerges in the nearly parallel light form in a main scanning section as it is. The light beam is converged in a sub-scanning section, and is focused as an almost linear image on a deflection facet (a reflective facet) 95a of a deflecting unit 95 comprised of a polygon mirror.
The light beam deflected by the deflection facet 95a of the deflecting unit 95 is transmitted through an scanning optical system 96 with the f-θ characteristic, and is projected on a scanned surface (a surface of a photosensitive drum) 98. The deflecting unit 95 is rotated in a direction indicated by an arrow A to optically scan the surface 98 of the photosensitive drum in a direction indicated by an arrow B to record image information thereon.
Further, in an optical scanning apparatus used in a color image forming apparatus, plural light source units are provided, and light beams are incident on a single or plural deflecting units. Here, the plural light beams are incident on the reflective deflection facet at different angles, are transmitted through a single or plural scanning optical systems, and are separated from each other by a mirror or the like. Plural light spots are hence scanned on different surfaces to be scanned.
In those optical scanning apparatuses, the diameter of the spot on the scanned surface needs to be decreased such that highly-precise recording of the image information can be achieved. For the purposes of decreasing the spot diameter, it is important to reduce wave front aberration caused by spherical aberration and the like. In a meridional (meridian) direction corresponding to the main scanning direction, reduction of aberrations is performed by using an aspherical surface, which is conventionally employed. In recent years, also with respect to a sagittal line corresponding to the sub-scanning direction, reduction of aberrations, particularly correction of the spherical aberration, has been proposed as disclosed in Japanese Patent Application Laid-Open Nos. H11-237569 and H11-271658.
It is, however, quite difficult to mold an anamorphic aspherical profile which has different aspherical shapes in both the main scanning direction and the sub-scanning direction. Its measurement and estimation are also difficult to perform.
On the other hand, in an optical scanning apparatus of an oblique-incidence type in which a light beam is obliquely incident on the deflection facet in the sub-scanning section, the deflection facet of the polygon mirror or the like moves back and forth at the time of deflection, and the light beam is incident on such deflection facet forming an angle relative thereto. Hence, a scanning line on the scanned surface is likely to curve.
Particularly, in a color image forming apparatus in which respective optical scanning apparatuses are provided for four photosensitive bodies on each of which a latent image is formed by each laser light, and images of originals of Y (yellow), M (magenta), C (cyan) and Bk (black) are formed on surfaces of corresponding photosensitive bodies, and four images of Y, M, C and Bk formed on the respective photosensitive bodies are superimposed on a transfer medium such as a sheet of paper. Therefore, if curvatures occur in scanning lines of the optical scanning apparatuses provided for the respective photosensitive bodies, deviation appears between the scanning lines of four colors. Chromatic deviation thus occurs in the image on the transfer medium, and quality of the image is likely to be extremely degraded.
For the purposes of solving the above problem, a method of decentering a portion of the scanning optical system in a direction perpendicular to the deflection facet to reduce the curvature of the scanning line is proposed as disclosed in Japanese Patent Application Laid-Open No. H07-191272. However, the following disadvantages exist. Since appearing fashions of the curvature in the scanning lines differ from each other depending on those oblique-incidence and decentering, it is difficult to achieve a complete correction in the entire range of the image. Since rotation of the spot on the scanned surface is remarkable due to the decentering in an scanning optical system wherein imaging in the sub-scanning direction is performed by a magnifying system, the above method can be applied only to an scanning optical system having a substantially reducing system.